Ion storage apparatus have been employed in a number of different applications in which control over the motions of ions is desired. In particular, ion storage apparatus have been utilized as mass analyzers or sorters in mass spectrometry (MS) systems. An ion storage apparatus includes an ion trap in which selected ions covering a wide range of differing mass-to-charge (m/z) ratios may be introduced or formed, stored for a desired period of time, and subjected to dissociation or other processes. Ions may also be selectively ejected from the ion trap to eliminate or detect the ejected ions, or to isolate other ions that are desired to be retained in the ion trap for additional study or processing. Depending on design, an ion trap may be established by electric and/or magnetic fields. Insofar as the present disclosure is concerned, the typical designs and operations of various types of ion storage apparatus, and various types of MS systems that employ ion storage apparatus, are generally known and need not be described in detail in the present disclosure.
In the operation of an ion storage device that provides an electric field-based ion trap, a radio frequency (RF) signal is applied to an electrode structure of the ion storage device to create an RF trapping field. The RF trapping field constrains the motions of ions along two or three dimensions to an ion trapping volume or region in the interior space of the electrode structure. A supplemental RF signal may also be applied to the electrode structure in combination with the main RF trapping signal to create a supplemental RF excitation field. The supplemental RF field may be utilized, among other purposes, to eject ions from the ion trapping volume for elimination or detection. In particular, the supplemental RF field may be utilized to eject unwanted ions from the ion trap and thereby isolate desired ions of a selected mass or range of masses in the ion trap. To isolate desired ions, it is possible to simultaneously eject all undesired ions over a range of differing m/z ratios from the ion storage apparatus by generating the excitation field from a supplemental RF signal having a broadband waveform. Moreover, the broadband waveform signal may have a notch in its frequency spectrum. Operating parameters may be set such that the secular frequency of a desired ion or ions falls within the bandwidth of the notch (the notch band). The notch band contains no signal components with a frequency corresponding to this secular frequency. Thus, the notch broadband waveform signal may be utilized to eject undesired ions whose masses are both greater and less than the mass of the desired ion, while the desired ion remains in the trap unaffected by this broadband signal and thus isolated from the ejected undesired ions.
The ion motion of two ions of different m/z ratios may be tightly coupled due to the characteristic or secular frequencies of the two ions being close to each other. This proximity of the secular frequencies of two different ions is problematic when a notch broadband waveform signal is applied to an ion storage apparatus to isolate an ion. Consider, for example, a plurality of ions that have been trapped in an ion storage apparatus. The ions include a desired ion having an m/z ratio of M, an undesired ion having an m/z ratio of M+1, and other ions having m/z ratios of M+i where i>1. A notch ejection waveform signal may be applied to the ion storage apparatus such that the secular frequency of the M ion falls in the frequency bandwidth of the notch (the notch band), the secular frequency of the M+1 ion falls outside the notch band but at or near the edge of the notch band, and the respective secular frequencies of the other M+i ions fall farther away from the notch band than the M+1 ion. More power is required to eject the M+1 ion than M+i ions. Conventionally, this requirement has been addressed by applying the entire composite waveform signal at a high enough average power to effectively eject the M+1 ion and thus separate the M+1 ion from the M ion. This means, however, that the high power is also employed to eject the more remote M+i ions. Unfortunately, this high power tends to reduce the effective bandwidth of the notch and consequently reduce the mass resolution. Moreover, the higher power required to effectively eject the M+1 ions is not likewise required to eject the other undesired (M+i) ions whose masses are more remote from the desired M ion.
In view of the foregoing, it would be advantageous to provide ion isolation waveform signals that are better tailored to isolate desired ions from undesired ions and do not require as much power as previously applied isolation waveform signals. These improved isolation waveform signals would provide high power only where it is needed—at frequencies at or close to the secular frequency corresponding to the desired ion to be isolated for use in resonantly ejecting ions of m/z ratios close to that of the desired ion, but not at the frequencies associated with undesired ions whose m/z ratios are more remote to that of the desired ion. In this manner, desired ions could be efficiently isolated from undesired ions while mass resolution is improved or at least not degraded, and the ion isolation signals could be applied with less average power than conventionally required.